The long range objective of this project is to characterize images of cells from human breast aspiration biopsies, and to distinguish malignant from benign cells. Aspiration biopsy is used widely in Europe but not in the United States. Biopsy material was prepared at the Department of Clinical Cytology, Academic Hospital, Uppsala, Sweden, stained with Papanicolau stain and scanned there, digitized and recorded on magnetic tape. The Uppsala scanning microscope has 7/10 micron square aperture and has a moving stage and records 256 gray levels. Using the PEEP system, cell images were displayed and thresholded. This generated objects which were subjected to feature extraction using the PEEP/DECIDE system. Eighteen numeric characterizing features were extracted for each of 100 cell nuclei in five cell categories. Examples of these features are area, average brightness, average chord, entropy, kurtosis and skewness of the optical density histogram, density, variation in brightness, variation in chord length and variation in diameter. Each of four categories of benign cells was distinguished from the category of malignant cells. Using quadratic discriminations, combinations of as few as four features and in some cases a single feature allowed perfect discrimination between pairs of cell categories. Useful features included skewness of the optical density histogram, nuclear area, length of major axis, density and entropy. The practical significance of this research lies largely to developments in the automation of cytologic examinations. The project will be continued both in the United States and Sweden using more carefully scanned cells prepared with gallocyanin chromalum.